Method and apparatus for designing matching network for eam for eml tosa

ABSTRACT

A transmitter optical sub-assembly (TOSA) with low group delay (GD) performance over an operating frequency range of the TOSA is designed based on a synthesis of low pass filters. The low pass filters include a first stage low pass filter (LPF 1 ) and a second stage low pass filter (LPF 2 ) coupled to the LPF 1  in a cascade form and a load impedance. The LPF 1  and the LPF 2  are configured to include inductance of stray components in the TOSA.

CROSS-REFERENCES AND RELATED APPLICATION(S)

This application claims priority to U.S. Provisional Application No. 62/724,890, filed Aug. 30, 2018, titled “METHOD AND SYSTEM FOR MITIGATING ADVERSE EFFECTS OF BONDING WIRE OF EXTERNAL OPTICAL MODULATORS,” the content of which is incorporated herein by reference in its entirety.

TECHNICAL FIELD

The present disclosure relates to design and methodology for matching an electro-absorption modulator (EAM) for electro-absorption modulator integrated laser (EML) transmitter optical sub-assembly (TOSA).

BACKGROUND

Bonding wires comprise an internal part of integrated circuit (IC) packaging for making connections to other circuitry, such as active devices including optical modulators and for input and output connections. As such, bonding wires are used extensively in packaging technology for chips. However, the bonding wires introduce extra parasitic inductance in the form of inductance in series with resistance at high frequencies. Further, the number of bonding wires, their heights from substrate, frequency and dimension may often play an important role in overall circuit performance.

The demand for high speed data transmissions or communications is ever increasing with more data intensive applications. At a high data rate, a transmitter optical sub-assembly (TOSA) becomes a major portion of the transceiver manufacturing cost. Further, for the higher data rate operation, there is a need for a careful design of a high frequency transmission structure of a TOSA package. By way of example, for an electro-absorption modulator integrated laser (EML) based TOSA, it may include in a package a high-speed electro-absorption (EAM) laser, a ceramic submount, a TEC, parasitic components, etc. in a small form factor coaxial package. As such, high frequency characteristics of the TOSA may be degraded by the parasitic components including bonding wires, bonding pads, EAM circuit parameters, etc.

Therefore, there is a further need for new and improved techniques for designing or packaging optical components of a TOSA.

SUMMARY

In various aspects of the present disclosure, a method for designing an electro-absorption modulator (EAM) matching network is designed for an electro-absorption modulator integrated laser (EML) on a submount in a transmitter optical sub-assembly (TOSA) for a high bit rate is disclosed herein. The method includes designing a first stage low pass filter (LPF1) including a first predetermined filter order and a first bandwidth (BW1), the LPF1 being coupled to a source impedance, and designing a second stage low pass filter (LPF2) including a second predetermined filter order and a second bandwidth (BW2). The LPF1 and LPF2 are configured to include inductance of stray components including bonding wires.

In an aspect of the present technology, designing the LPF2 may include designing the LPF2 such that the second bandwidth (BW2) of the LPF2 is wider than the first bandwidth (BW1) of the LPF1 to not interfere with performance of the LPF1.

In another aspect of the present technology, the LPF2 may be configured to act as a signal reflector to compensate a frequency response of the LPF1.

In another aspect of the present technology, the source impedance and the load impedance may be set to 50 Ohms respectively.

In another aspect of the present technology, designing the LPF2 may include determining the second bandwidth (BW2) of the LPF2 based on a requirement of the TOSA.

In another aspect of the present technology, designing the LPF2 may further include selecting a filter type of the LPF2.

In another aspect of the present technology, the method may further include controlling the second bandwidth (BW2) of the LPF2 to adjust an amount of a reflected signal into an output of the LPF2.

In an aspect of the present technology, the LPF1 and LPF2 may comprise Bessel filters of a predetermined filter order.

In an aspect of the present technology, the LPF1 may include a 4^(th) order Bessel low pass filter and the LPF2 may include a 2^(nd) order Bessel low pass filter.

In an aspect of the present technology, the second bandwidth (BW2) of the LPF2 may be about four (4) times the first bandwidth (BW1) of the LPF1.

In an aspect of the present technology, the high bit rate may comprise a bit rate greater than 25 Giga bits per second (Gb/s).

In an aspect of the present technology, a filter type of the LPF1 and/or LPF2 may be selected based on minimum group delay (GD) performance.

In an aspect of the present technology, the LPF1 and/or the LPF2 may include a Bessel filter or a Linear Phase Equi-ripple Error filter.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects and advantages of the present disclosure will become better understood from the following description, appended claims, and accompanying figures where:

FIG. 1 is a schematic diagram conceptually illustrating an embodiment of the present technology in accordance with an aspect of the present disclosure;

FIG. 2 is a schematic diagram conceptually illustrating an embodiment of the present technology in accordance with an aspect of the present disclosure;

FIG. 3 is a flowchart conceptually illustrating an embodiment of the present technology in accordance with an aspect of the present disclosure;

FIG. 4 is a schematic diagram illustrating an example embodiment of the present technology in accordance with an aspect of the present disclosure;

FIGS. 5A-5C are examples of simulation results conceptually illustrating an embodiment of the present technology in accordance with an aspect of the present disclosure;

FIGS. 6A-6F are examples of simulation results conceptually illustrating an embodiment of the present technology in accordance with an aspect of the present disclosure;

FIG. 7 illustrates simulation results of an embodiment of the present technology in accordance with an aspect of the present disclosure;

FIG. 8 illustrates simulation results of an embodiment of the present technology in accordance with an aspect of the present disclosure;

FIG. 9 illustrates simulation results of an embodiment of the present technology in accordance with an aspect of the present disclosure;

FIG. 10 is a schematic diagram illustrating an example embodiment of the present technology in accordance with an aspect of the present disclosure;

FIGS. 11A-11C illustrate simulation results of an embodiment of the present technology in accordance with an aspect of the present disclosure;

FIGS. 12A-12F are examples of simulation results conceptually illustrating an embodiment of the present technology in accordance with an aspect of the present disclosure;

FIG. 13 illustrates simulation results of an embodiment of the present technology in accordance with an aspect of the present disclosure;

FIG. 14 illustrates simulation results of an embodiment of the present technology in accordance with an aspect of the present disclosure;

FIG. 15 illustrates simulation results of an embodiment of the present technology in accordance with an aspect of the present disclosure;

FIG. 16 illustrates simulation results of an embodiment of the present technology in accordance with an aspect of the present disclosure;

FIG. 17 illustrates simulation results of an embodiment of the present technology in accordance with an aspect of the present disclosure; and

FIG. 18 illustrates simulation results of an embodiment of the present technology in accordance with an aspect of the present disclosure.

DETAILED DESCRIPTION

The detailed description of illustrative examples will now be set forth below in connection with the various drawings. The description below is intended to be exemplary and in no way limit the scope of the present technology. It provides a detailed example of possible implementation and is not intended to represent the only configuration in which the concepts described herein may be practiced. As such, the detailed description includes specific details for the purpose of providing a thorough understanding of various concepts, and it is noted that these concepts may be practiced without these specific details. In some instances, well known structures and components are shown in block diagram form in order to avoid obscuring such concepts. It is noted that like reference numerals are used in the drawings to denote like elements and features.

Further, methods and devices that implement example embodiments of various features of the present technology are described herein. Reference in the description herein to “one embodiment” or “an embodiment” is intended to indicate that a particular feature, structure, or characteristic described in connection with the example embodiments is included in at least an embodiment of the present technology or disclosure. The phrases “in one embodiment” or “an embodiment” in various places in the description herein are not necessarily all referring to the same embodiment.

In the following description, specific details are given to provide a thorough understanding of the example embodiments. However, it will be understood by one of ordinary skill in the art that the example embodiments may be practiced without these specific details. Well-known circuits, structures and techniques may not be shown in detail in order not to obscure the example embodiments (e.g., circuits in block diagrams, etc.).

Bonding wires comprise an internal part of integrated circuit (IC) packaging for making connections to other circuitry, such as active devices including optical modulators and for input and output connections. As such, bonding wires are used extensively in packaging technology for chips. However, the bonding wires introduce extra parasitic inductance in the form of inductance in series with resistance at high frequencies. Further, the number of bonding wires, their heights from substrate, frequency and dimension may often play an important role in overall circuit performance. Further, extra capacitance may be introduced due to bonding pads.

Maintaining linearity of optical devices as well as electronic devices are indispensable. In particular, to maintain the signal integrity, the total amount of harmonic distortion may be controlled by designing an integrated circuit (IC) to have minimum distortion or optimizing a modulation index of electro-absorption modulator integrated lasers (EMLs) and directly modulated lasers (DMLs). Also, there is another important parameter to be considered, but quite often being neglected until now, which is group delay (GD) variations over an operating frequency range to be used for modulation. When the GD variation is high, an output signal of a device may be distorted. As such, it will be important to maintain the GD variation constant and low when designing a transmitter optical sub-assembly (TOSA) using an EML or DML external modulator.

When EMLs or DMLs are packaged in a TOSA, the GD variation can be significantly affected by stray electronic components such as bonding wires, bonding pads, equivalent circuit components of EMAs, etc. In the design of a device for higher order signals, the GD has become a key circuit parameter of a TOSA for higher order modulation signals like PAM4, DMT, etc. because the GD affects the distortion of modulated optical signals. Up until now, there has been no clear guidelines showing how to package an EML or a DML in a TOSA to meet a certain requirement of the TOSA by taking the GD into account. That is, the present technology disclosed herein provides a design methodology and technique for packaging the EML or DML in a TOSA, taking the GD into account, thereby improving the performance of the TOSA.

FIG. 1 is a circuit diagram conceptually illustrating a simplied equivalent circuit of a TOSA assembly including an EML. In the example, inductance L1 and L2 represent inductance of bonding wires, R_(S) represents a source impedance, R_(L) represents a load impedance, C-EAM represents capacitance of EAM and R_(ph) represents photo induced equivalent resistance. As mentioned above, at a high bit rate, small parasitic components caused by parasitic elements or components such as bonding wires and pads affect the frequency response and performance of a EML-based TOSA. More specifically, as the bit rate goes up greater than 25 Gb/s, the control of parasitic components involved in the TOSA assembly and equivalent circuit of EAM becomes very important in the packaging.

FIG. 2 illustrates an embodiment of the present technology, in an aspect of the present disclosure, showing a first stage low pass filter having an m-th order and a second stage low pass filter having an n-th order. Further, the first stage low pass filter and the second stage low pass filter may be synthesized and designed with a specific performance requirement of a TOSA. In the example of FIG. 2, an equivalent circuit of an EML packaged in a TOSA includes a first stage low pass filter (LPF1) and a second stage low pass filter (LPF2). Further, from a perspective of the LPF1, the photo-induced resistance (R_(ph)) is considered to be part of the load resistance and from a perspective of the LPF2, the photo-induced resistance (R_(ph)) is considered to be part of the source resistance of LPF2. Also, R_(L) is the load resistance of LPF2 and becomes the load resistance of the entire low pass filter.

FIG. 3 illustrates a flowchart conceptually representing a design process in an aspect of the present disclosure. As shown in FIG. 3, at S301, a first stage low pass filter (LPF1) is designed and synthesized with a source impedance of 50 Ohm and a load impedance of 50 Ohm for an EML in the TOSA. In an aspect of the present disclosure, once capacitance of EAM (e.g., C-EAM) is known, LPF1 may be designed in such a way that a certain type of LPF1 includes the same value of C-EAM. In another aspect of the present disclosure, if there is a bandwidth requirement for the TOSA (or to meet a certain bandwidth of TOSA to be designed), LPF1 may be designed to meet the bandwidth requirement.

Further, in one example, a 3^(rd) order Bessel filter with a first bandwidth (BW1) may be used for LPF1. In an aspect of the present disclosure, a 4^(th) order Bessel filter may be used for the first stage LPF1 and a 2^(nd) order Bessel filter may be used for the second stage LPF2. Still in another aspect of the present disclosure, a 2^(nd) order Bessel filter may be used for the first stage LPF1 and a 2^(nd) order Bessel filter may be used for the second stage LPF2.

At S303, after designing the first stage low pass filter LPF1, a second stage low pass filter (LPF2) may be designed with a second bandwidth (BW2). In the example, LPF2 may be designed to separate photo-induced resistance (R_(ph)) and termination resistance (R_(L)) and be in cascade with LPF1. In one implementation, the second bandwidth (BW2) of LPF2 may be wider than the first bandwidth (BW1) of LPF1 such that the first bandwidth (BW1) and the second bandwidth (BW2) do not interfere the performance of LPF1 when LPF1 and LPF2 are cascaded. Further, in another aspect of the present disclosure, a Bessel filter may be selected for LPF2 and the second bandwidth (BW2) of LPF2 may be set to about four (4) times the first bandwidth (BW1) of LPF1.

At S305, the second bandwidth (BW2) and/or filter type of LPF2 may be adjusted to meet certain performance requirements of EML TOSA (e.g., a bandwidth, group delay, etc.). That is, according to the purpose of the TOSA design, the second bandwidth (BW2) of LPF2 may be determined and the filter type of LPF2 may be selected to fit with the best performance of the TOSA.

In an aspect of the present disclosure, the second stage low pass filter LPF2 may be designed to act as a signal reflector to compensate a frequency response of LPF1. That is, by controlling the second bandwidth (BW2) of LPF2, the amount of the reflected signal may be adjusted. By doing so, the electrical-to-optical (EO) response of EAM may be adjusted and improved to a wider bandwidth.

In an aspect of the present disclosure, the second bandwidth (BW2) of the TOSA that is to be developed may be adjusted by changing the second bandwidth (BW2) and/or filter type of LPF2. In one example, the filter type of LPF2 may be a Bessel filter, a Butterworth filter or the like. Further, the second bandwidth (BW2) of LPF2 may be adjusted by changing the termination resistance or the load resistance (R_(L)). In one implementation, the load resistance may be set to 50 Ohm, 30 Ohm, 25 Ohm or the like. Furthermore, a performance metric such as group delay (GD) may be analyzed and a filter type of LPF2 may be selected accordingly. In one implementation, for minimum GD performance, either a Bessel filter or a Linear Phase Equi-ripple Error Filter may be selected as the LPF2.

FIG. 4 is an example circuit diagram for simulating a matching network for an EML-based TOSA in an aspect of the present disclosure. In the example, the first stage low pass filter (LPF1) is a 4^(th) order Bessel filter including inductance L1 and L3, capacitance C2, and capacitance C-EAM, and the second stage low pass filter (LPF2) is a 2^(nd) order Bessel filter including inductance L5 and capacitance C4. Other simulation parameters for the example circuit are shown in FIG. 4. Further, in an aspect of the present disclosure, LPF2 is designed such that the second bandwidth (BW2) of LPF2 is about four (4) times the first bandwidth (BW1) of LPF1.

In an aspect of the present disclosure, FIGS. 5-9 show various simulation results of the example circuit of FIG. 4. FIGS. 5A-5C show a frequency response and a group delay response of the example circuit of FIG. 4. That is, FIGS. 5A-5C show simulation results of the matching network: the first stage low pass filter (LPF1) of a second order Bessel filter and the second stage low pass filter (LPF2) of a second order Bessel filter. As shown in FIG. 5A and FIG. 5C, the −3 dB bandwidth of the system is about 33.9 GHz and the group delay (GD) is about 1.1 picoseconds (ps) over an operating frequency range, e.g., 33.9 GHz. Further, it is noted that the −3 dB bandwidth of 33.9 GHz is due to the back reflection from the second stage low pass filter (LPF2).

FIGS. 6A-6F show simulation results when the second bandwidth (BW2) of the second stage low pass filter (LPF2) is changed from 90 GHz to 60 GHz. In the example, FIGS. 6A-6C show the frequency responses and group delay response when the second stage low pass filter (LPF2) is set at 90 GHz, and FIGS. 6D-6F show the frequency responses and group delay response when the second stage low pass filter (LPF2) is set at 60 GHz. At 90 GHz, the −3 dB bandwidth (e.g., S21 response) is observed to be 35.9 GHz and the group delay over the −3 dB bandwidth is observed to be about 2 ps over 35.9 GHz. At 60 GHz, the −3 dB bandwidth (e.g., S21 response) is observed to be 36.6 GHz and the group delay over the −3 dB bandwidth is observed to be about 3.01 ps.

FIGS. 7-9 illustrate various simulation results when series resistance, R_(ph) or C-EAM is varied. FIG. 7 shows frequency responses (e.g., S11 and S21) and group delay response when the serial resistance is varied from 0 to 10 Ohm, e.g., 0-10 Ohm, and the photo-induced resistance R_(ph) is fixed at 125 Ohm and the capacitance of EAM (C-EAM) is fixed at 0.2378 pF. As seen in FIG. 7, the group delay varies about 2 ps, as the serial resistance varies from 0 to 10 Ohm.

FIG. 8 shows frequency responses (e.g., S11 and S21) and group delay response when the serial resistance is fixed at 5 Ohm, the capacitance of EAM is fixed at 0.2378 pF, and the photo-induced resistance R_(ph) is varied from 100 Ohm to 150 Ohm. As can be seen in FIG. 8, the group delay is observed to be less than 1 ps up to 30 GHz as the photo-induced resistance R_(ph) is varied from 100 Ohm to 150 Ohm.

FIG. 9 shows frequency responses (e.g., S11 and S21) and group delay response when the serial resistance is fixed at 5 Ohm, the photo-induced resistance R_(ph) is fixed at 125 Ohm, and the capacitance of EAM is varied from 0.2 pF to 0.26 pF. As can be seen in FIG. 9, the group delay is observed to be less than 2 ps up to 30 GHz as the capacitance of EAM is varied from 0.2 pF to 0.26 pF.

FIG. 10 is another circuit diagram for simulating a matching network for a EML-based TOSA in an aspect of the present disclosure. In the example, the first stage low pass filter (LPF1) comprises a 2^(th) order Bessel filter including inductance L3, capacitance C2, and capacitance C-EAM, and the second stage low pass filter (LPF2) comprises a 2^(nd) order Bessel filter including inductance L5 and capacitance C4. Other simulation parameters for the example circuit are shown in FIG. 10. In the example, LPF2 is designed such that the second bandwidth (BW2) of LPF2 is set to about four (4) times the first bandwidth (BW1) of LPF1.

In an aspect of the present disclosure, FIGS. 11-15 show various simulation results of the example circuit of FIG. 10. FIG. 11 shows frequency response and group delay response of the example circuit of FIG. 10. That is, FIG. 11 shows simulation results of the matching network: the first stage low pass filter (LPF1) of a second order Bessel filter and the second stage low pass filter (LPF2) of a second order Bessel filter. As shown in FIG. 11, the −3 dB bandwidth of the system is about 34.4 GHz and the group delay (GD) is about 1.05 picoseconds (ps) over the operating frequency range or the −3 dB bandwidth of 34.4 GHz. Further, it is noted that the −3 dB bandwidth of 34.4 GHz is due to the back reflection from the second stage low pass filter (LPF2).

FIG. 12 shows simulation results when the second bandwidth (BW2) of the second stage low pass filter (LPF2) is changed. In the example, FIGS. 12A-12C show the frequency responses and group delay response when the second stage low pass filter (LPF2) is fixed at 90 GHz, and FIGS. 12D-12F show the frequency responses and group delay response when the second stage low pass filter (LPF2) is set at 60 GHz. At 90 GHz, the −3 dB bandwidth (e.g., S21 response) is observed to be 35.1 GHz and the group delay over the −3 dB bandwidth is observed to be about 1.25 ps over 35.1 GHz. At 60 GHz, the −3 dB bandwidth (e.g., S21 response) is observed to be 35.3 GHz and the group delay over the −3 dB bandwidth is observed to be about 2.05 ps over 35.3 GHz.

FIG. 13 shows frequency responses (e.g., S11 and S21) and group delay response when the serial resistance is varied, e.g., 0-10 Ohm, and the photo-induced resistance R_(ph) is fixed at 125 Ohm and the capacitance of EAM (C-EAM) is fixed at 0.228 pF. As seen in FIG. 13, the group delay difference of about 2 ps is observed as the serial resistance is varied from 0 to 10 Ohm.

FIG. 14 shows frequency responses (e.g., S11 and S21) and group delay response when the serial resistance is fixed at 5 Ohm, the capacitance of EAM is fixed at 0.228 pF, and the photo-induced resistance R_(ph) is varied from 100 Ohm to 150 Ohm. As can be seen in FIG. 14, the group delay is observed to be less than 1 ps up to 30 GHz as the photo-induced resistance R_(ph) is varied from 100 Ohm to 150 Ohm.

FIG. 15 shows frequency responses (e.g., S11 and S21) and group delay response when the serial resistance is fixed at 5 Ohm, the photo-induced resistance R_(ph) is fixed at 125 Ohm, and the capacitance of EAM is varied from 0.2 pF to 0.26 pF. As can be seen in FIG. 15, the group delay is observed to be about 2 ps up to 30 GHz as the capacitance of EAM is varied from 0.2 pF to 0.26 pF. As can be noted above, the group delay is well confined within a reasonable variation range.

In an aspect of the present disclosure, a few observations may be made as follows. By way of example, as for the series resistance, S21 bandwidth may be affected by the series resistance and it is the smaller, it is the better. As for the photo-induced resistance, it is noted to be less sensitive. As for the modulator capacitance (e.g., C-EAM) is concerned, it is the smaller, it is the better. Thus, it may be important to design an EAM such that the EAM has smaller capacitance.

In one implementation, the modulator capacitance may be selected for meeting a certain bandwidth requirement of the TOSA over a frequency range, using a 2^(nd) order Bessel LPF, as follows.

Frequency Capacitance (pF) 2.50 GHz 2.736 5.00 GHz 1.368 7.50 GHz 0.912 10.0 GHz 0.684 12.5 GHz 0.547 15.0 GHz 0.456 17.5 GHz 0.391 20.0 GHz 0.342 22.5 GHz 0.304 25.0 GHz 0.274 27.5 GHz 0.249 30.0 GHz 0.228 32.5 GHz 0.210 35.0 GHz 0.195 37.5 GHz 0.182 40.0 GHz 0.171

Further, in an aspect of the present disclosure, the above table values for the capacitance of an EAM may be obtained by the following expression: C_(EAM) (pF)=6.8/f (GHz), where f=frequency.

Further, referring back to FIG. 10, in another aspect of the present disclosure, it is noted that as the inductance L5 (as in FIG. 10) increases, the bandwidth also increases to some extent at a cost of the increased group delay. This effect may be shown with the decreased second bandwidth (BW2) of the second stage low pass filter (LPF2). As such, in order to decrease the second bandwidth (BW2) of LPF2, the inductance L5 should be increased.

In another aspect of the present disclosure, when the termination resistance or load resistance of 50 Ohm (e.g., a source impedance of 50 Ohm and a load impedance of 50 Ohm), the bandwidth may be increased to 38.2 GHz from 30 GHz. In such as case, FIG. 16 shows corresponding frequency responses and group delay response. In the example, the second bandwidth (BW2) of the second stage low pass filter LPF2 is about 120 GHz. It is noted that low frequency S11 is significantly increased while low frequency S21 is decreased by 1.6 dB from −13.2 dB to −14.8 dB. Also, the group delay is observed to be about 1.27 ps up to 38.2 GHz.

In another aspect of the present disclosure, FIGS. 17 and 18 show corresponding frequency and group delay responses, when the termination resistance or load resistance of 25 Ohm (e.g., a source impedance of 50 Ohm and a load impedance of 25 Ohm), the bandwidth is increased to 55 GHz from 30 GHz. FIG. 17 shows the frequency and group delay response when the second bandwidth of the second stage low pass filter LPF2 is about 120 GHz and FIG. 18 shows the frequency and group delay response when the second bandwidth of the second stage low pass filter LPF2 is about 160 GHz. It is noted that low frequency S11 is significantly increased while low frequency S21 is decreased by 4.8 dB from −13.2 dB to −18 dB. Also, the group delay is observed to be about 1.27 ps up to 38.2 GHz when the second bandwidth (BW2) is set at 120 GHz. Further, as noted, when the second bandwidth (BW2) is set at 120 GHz, the group delay is observed to be about 6 ps up to 55 GHz, whereas when the second bandwidth (BW2) is set at 160 GHz, the group delay is observed to be about 3 ps up to 55 GHz.

As such, in various aspects of the present disclosure, the present technology disclosed herein provide much improved performance including frequency responses and group delay responses by means of designing a matching network of an EML in a TOSA, based on synthesis of a first stage low pass filter LPF1 and a second stage low pass filter LPF2, alone or in combination of each other.

As used in the present, except explicitly noted otherwise, the term “comprise” and variations of the term, such as “comprising,” “comprises,” and “comprised” are not intended to exclude other additives, components, integers or steps.

The terms “first,” “second,” and so forth used herein may be used to describe various components, but the components are not limited by the above terms. The above terms are used only to discriminate one component from other components, without departing from the scope of the present disclosure. Also, the term “and/or” used herein includes a combination of a plurality of associated items or any item of the plurality of associated items. Further, it is noted that when it is described that an element is “coupled” or “connected” to another element, the element may be directly coupled or directly connected to the other element, or the element may be coupled or connected to the other element through a third element. A singular form may include a plural form if there is no clearly opposite meaning in the context. In the present disclosure, the term “include” or “have” used herein indicates that a feature, an operation, a component, a step, a number, a part or any combination thereof described herein is present. Further, the term “include” or “have” does not exclude a possibility of presence or addition of one or more other features, operations, components, steps, numbers, parts or combinations. Furthermore, the article “a” used herein is intended to include one or more items. Moreover, no element, act, step, or instructions used in the present disclosure should be construed as critical or essential to the present disclosure unless explicitly described as such in the present disclosure.

Although the present technology has been illustrated with specific examples described herein for purposes of describing example embodiments, it is appreciated by one skilled in the relevant art that a wide variety of alternate and/or equivalent implementations may be substituted for the specific examples shown and described without departing from the scope of the present disclosure. As such, the present disclosure is intended to cover any adaptations or variations of the examples and/or embodiments shown and described herein, without departing from the spirit and the technical scope of the present disclosure. 

What is claimed as:
 1. A method for designing an electro-absorption modulator (EAM) matching network for an electro-absorption modulator integrated laser (EML) on a submount in a transmitter optical sub-assembly (TOSA) configured to operate at a high bit rate, the method comprising: designing a first stage low pass filter (LPF1) with a first predetermined filter order and a first bandwidth (BW1), the LPF1 being coupled to a source impedance (R_(S)); and designing a second stage low pass filter (LPF2) coupled to the LPF1 in a cascade form and coupled to a load impedance (R_(L)), the LPF2 having a second predetermined filter order and a second bandwidth (BW2), wherein the LPF1 and the LPF2 are configured to include inductance of stray components in the TOSA.
 2. The method of claim 1, wherein designing a first stage low pass filter (LPF1) comprises designing the LPF1 with the source impedance and the load impedance of 50 Ohm.
 3. The method of claim 1, wherein the stray components comprise bonding wires on the submount, bonding pads and equivalent components of EAM.
 4. The method of claim 1, wherein designing a second stage low pass filter (LPF2) comprises designing the LPF2 such that the second bandwidth (BW2) of the LPF2 is wider than the first bandwidth (BW1) of the LPF1 so as for the BW1 and BW2 to not interfere with performance of the LPF1.
 5. The method of claim 1, further comprising adjusting a bandwidth of the TOSA by: (i) changing the second bandwidth (BW2) and a filter type of the LPF2; (ii) changing a value of the load resistance; or (iii) selecting a Bessel filter or a Linear Phase Equi-ripple filter based on minimum group delay (GD) performance.
 6. The method of claim 1, wherein the LPF1 is configured such that photo-resistance (R_(ph)) is part of the load impedance (R_(L)) and the LPF2 is configured such that the photo-resistance (R_(ph)) is part of the source impedance (R_(S)).
 7. The method of claim 1, wherein the LPF2 is configured to act as a signal reflector to compensate a frequency response of the LPF1.
 8. The method of claim 1, wherein designing the LPF2 comprises determining the second bandwidth (BW2) of the LPF2 based on performance requirements of the TOSA.
 9. The method of claim 8, wherein designing the LPF2 further comprises selecting a Bessel filter type for the LPF2.
 10. The method of claim 1, further comprising controlling the second bandwidth (BW2) of the LPF2 to adjust an amount of a reflected signal.
 11. The method of claim 10, wherein controlling the second bandwidth (BW2) of the LPF2 comprises adjusting an electrical-to-optical (EO) response of the EAM to a wider bandwidth.
 12. The method of claim 1, wherein both of the LPF1 and the LPF2 comprise Bessel filters of a predetermined filter order.
 13. The method of claim 12, wherein the LPF1 comprises a 4^(th) order Bessel low pass filter and the LPF2 comprises a 2^(nd) order Bessel low pass filter.
 14. The method of claim 12, wherein the LPF1 comprises a 2^(nd) order Bessel low pass filter and the LPF2 comprises a 2^(nd) order Bessel low pass filter.
 15. The method of claim 1, wherein the second bandwidth (BW2) of the LPF2 is about four (4) times the first bandwidth (BW1) of the LPF1.
 16. The method of claim 1, wherein the high bit rate comprises a bit rate greater than 25 Giga bits per second (Gb/s).
 17. The method of claim 1, wherein a filter type of the LPF1 and/or the LPF2 is selected based on minimum group delay (GD) performance.
 18. The method of claim 17, wherein the LPF1 and/or the LPF2 comprise a Bessel filter or a Linear Phase Equi-ripple Error filter.
 19. A method of designing a transmitter optical sub-assembly (TOSA) with low group delay (GD) performance over an operating frequency range of the TOSA, based on a synthesis of low pass filters, wherein the low pass filters comprise a first stage low pass filter (LPF1) and a second stage low pass filter (LPF2) coupled to the LPF1 in a cascade form and a load impedance, and wherein the LPF1 and the LPF2 include inductance of stray components in the TOSA.
 20. The method of claim 19, wherein inductance and capacitance values obtained from the synthesis are set to capacitance of electronic-absorption modulator (EAM) and inductance of bonding wires on a submount in the TOSA. 